A Southwest Airlines plane, seen here April 4, sits in a remote area of the international airport in Yuma, Ariz., after a section of fuselage tore away during a flight on Friday. Three more Southwest Airlines jetliners have small, subsurface cracks similar to the ones suspected in the fuselage tear on this plane, the airline announced Monday.

An incident in which a Southwest Airlines jet tore open during flight has raised questions about the reliability of industry procedures used to test older airplanes for invisible cracks. The jet was able to land safely Friday, but Southwest was still scrambling Monday to test its planes.

While Southwest is the focus of attention, some are asking whether airplanes operated by other companies could also be vulnerable to similar failures.

Friday's accident gives no cause for concern about the structural integrity of airplanes in their early years of service, say some air safety experts. The Southwest airplane that departed from Phoenix Friday was an older one, as measured by the number of its takeoff and landing cycles.

The Federal Aviation Administration (FAA) said Monday it will issue an emergency safety order requiring special inspections of about 175 older 737s worldwide. About 80 of the planes are registered in the US, with nearly all of those operated by Southwest.

“Safety is our number one priority,” said Transportation Secretary Ray LaHood. “Last Friday’s incident was very serious and could result in additional action, depending on the outcome of the investigation.”

Southwest had already begun inspecting a portion of its fleet of 548 aircraft – all of which are Boeing 737s. The airline has found sub-surface cracks in three other planes, it said Monday. After further testing, those may need repairs before returning to service.

"What's surprising in this case," says safety expert William Voss, was that a "relatively well-proven [inspection system] either suddenly came up short, or something was done improperly. We don't know what yet."

Earlier this year, the FAA implemented new rules requiring additional structural inspections of Boeing 757 and 737 aircraft. The agency rejected Southwest's request for more time to complete inspections, which the company said would require "out-of-sequence maintenance" that would cause "a significant burden."

The FAA said its new directive will require inspections using electromagnetic technology in specific areas of the aircraft fuselage, on certain 737s that have been through more than 30,000 flight cycles. It will then require repeat inspections at regular intervals.

A significant regimen of fuselage testing has been in place for 737s, in particular, since an Aloha Airlines 737-200 ripped open during a 1988 flight, says Mr. Voss, who is president of the Flight Safety Foundation, a nonprofit group focusing on aviation safety issues. The plane landed safely but one flight attendant died during the incident.

In Friday's incident, the cabin depressurized and a sizable hole opened in the roof of the plane, prompting an emergency landing at an Arizona military base. Passengers said they could see blue sky through the roof of Flight 812, bound for Sacramento, Calif.

Aircraft in general are inspected with greater care as they age. Each takeoff and landing puts stress on a plane, due to the changes in cabin pressure as the plane ascends and descends. When so-called "fatigue cracks" are found, the planes are routinely repaired. The plane involved in Friday's incident was 15 years old, according to FAA records. On average, Southwest's fleet of 548 planes is 11 years old, each flying about six flights per day.

Voss says that Southwest accounts for most of the "high-cycle" 737s operating in the US. Those are planes that, like the one on the Phoenix flight, have been through as many as 39,000 takeoff and landing cycles.

Southwest has had problems with fuselages tearing in the past. In 2009, a foot-long hole opened in the top of a jet, forcing an emergency landing in West Virginia. That same year, the airline was fined $7.5 million by the FAA for nearly 60,000 flights in which the planes had not undergone required inspections for fuselage cracks.

Cracks caused by structural fatigue can be invisible to the eye. The current inspection of Southwest airplanes, aided by personnel from Boeing, uses a low-frequency current to analyze the aircraft skin for weakness.

Southwest cancelled a number of flights over the weekend, but said Monday it was operating all but about 70 of its 3,400 scheduled departures. The National Transportation Safety Board is investigating what happened on Southwest Flight 812.